D.L.
I too am obese and think while yes, it's best to lose weight prior to getting pregnant, it's far more important to get healthy. If you start too rigorous of an exercise program, you can actually make it harder to conceive. You should work on prepping your body for pregnancy with good eating habits, moderate exercise/activities, no drinking/smoking, and start taking prenatal vitamins now. You (and your husband) should prep your bodies for 3 months and then start "trying" the 4th month. It takes 90 days for your body to renew itself, and you should be healthy and ready to go by the 4th cycle. :-) I highly recommend the books "Taking Charge of Your Fertility" and "Natural Fertility". They both have great information on how to prep your body for conceiving.
The main thing is that if you do get pregnant while being obese, you absolutely don't want to gain much (if any) weight while pregnant. I (luckily) only was 9 pounds up from my starting weight at full-term and the most I ever gained was 11 pounds. By the time I had my 6-week check-up, I had lost 22 pounds, 13 less than before I was pregnant. So, I had actually lost fat weight while pregnant. Unfortunately, I gained the weight back once out little girl self-weaned at 9 1/2 months.
Little things make a BIG difference. My husband and I started making small changes last winter (first, no eating/drinking any calories after a certain time in the evenings, next month - no soda in the house, finally - no fast food). By the time the third month of changes came, we really started losing weight. My husband has lost about 40 pounds (maybe more now) because he started really working out this summer. I've lost about 18 pounds and already feel much better. My point is, start with your health and the weight will follow. Even a few pounds makes a difference, so hopefully you'll lose some weight by improving your health. :-D
Good luck! I'll be rooting for you. :-)